Add integration with TravisCI

Please configure integration with TravisCI:

[x] Enable repo testing in TravisCI
[x] Add TravisCI badge (master branch) to README.md with a single commit. (click next to octocat icon on this page)
[x] Create new branch called feature/travis-ci-integration and do all the following steps within that branch.
[x] Add .travis.yml using pre-commit as a script. Set it up to run Python 3.6 env and add PIP caching. Ensure you install pre-commit before running tests:
```
language: python
python: 3.6
cache: pip
before_install: pip install -U pre-commit
script: pre-commit run --all-files
```
[x] Push the branch and create a Pull Request for it.

kpi-web-guild / django-girls-blog-serhii73